Geology 200

Study Guide

Unit 14:
Plate Tectonics

Overview

The concept of plate tectonics was first described in Unit 1 and applied in later units to explain the origins of different rock types. This unit outlines how the theory of plate tectonics arose and developed. The theory of plate tectonics is an excellent illustration of how the scientific method works: collection of data; development of a hypothesis based on that data; testing, modifying, and rejecting the hypothesis; and developing a new hypothesis based on new data. Note: As you do the reading assignments for the section that relates to this, you must be careful to distinguish between the terms hypothesis (tentative explanation) and data (information used to evaluate a hypothesis). The data do not change, but a number of hypotheses can be developed to explain the same data, and hypotheses do change.

Unit 14 is divided into five sections. Section 1 discusses the precursor to the theory of plate tectonics: continental drift. Section 2 reviews the features of the sea floor that reflect plate tectonic activity. Section 3 discusses the concept of converging boundaries, and Section 4 discusses transform boundaries. Section 5 closes the unit with the causes of plate motions.

Objectives

After completing this unit, you should be able to

1. outline four lines of evidence, derived from Wegener’s study of the continents, that support the theory of continental drift.
2. describe the origin of magnetic patterns on the sea floor.
3. describe how basaltic magma is formed at spreading centres.
4. describe (using illustrations) the composition of the Earth’s crust.
5. list three types of plate boundaries, and describe (using illustrations) the processes occurring at each.
6. describe the relationship between transform faults and differential spreading rates.
7. describe three possible driving mechanisms for plate motion.
8. describe three models for mantle convection.
9. define the term mantle plume, and explain how mantle plumes are considered a driving mechanism for plate motion.
10. describe how mid-plate volcanic chains, such as the Hawaiian Island chain, are formed.
11. describe how plate tectonic theory explains the major tectonic features of the Earth.

Section 1: Continental Drift

The following reading describes an important precursor of the theory of plate tectonics: continental drift, or the idea that continents move freely over the Earth’s surface, changing their positions relative to one another. Alfred Wegener made a strong case for continental drift in the early 1900s. Paleomagnetic studies later provided renewed support for the theory.

Reading Assignment

Plummer, C. C., Carlson, D. H., & Hammersley, L. Physical Geology (15th ed.).

• Introduction to Chapter 19: Plate Tectonics – The Unifying Theory (pp. 453-455).
• “The Early Case for Continental Drift” (pp. 455-458).
• “The Revival of Continental Drift” (pp. 458-460).

Study Questions

1. What is continental drift?
2. What is seafloor spreading?
3. What is Pangaea?
4. List four lines of evidence derived from Wegener’s study of the continents that support the theory of continental drift.
5. Describe how paleomagnetic data support the theory of continental drift, rather than supporting polar wandering.

 Answer Key

Section 2: Oceanic Crust, the Sea Floor, Seafloor Spreading, and Divergent Plate Margins

Much of the data for understanding plate tectonics has been obtained through the study of the sea floor. The textbook devotes an entire chapter (Chapter 18) to describing the features of the sea floor. We encourage you to read it for interest, although it will not be assigned reading for this course.

In this section, we will examine the composition of the oceanic crust and the characteristics of mid-oceanic ridges, seamounts, guyots, and aseismic ridges.

Plate tectonic theory emphasizes the difference in composition between the continental crust and the oceanic crust. The greater density of the oceanic crust allows it to sink below the continental crust at collision boundaries. Oceanic crust is about seven kilometres thick, and is divided into three layers. The top layer consists of marine sediment with an average thickness of 0.5 km. The second layer is about 1.5 km thick, and consists mostly of pillowed and fractured basalt. The third layer is about 5 km thick, and consists of parallel sheeted dikes in the upper section and sill-like gabbro bodies in the lower part.

The study of ophiolite sequences has greatly aided the interpretation of the composition of the oceanic crust. Ophiolites are slivers of the oceanic crust and the upper mantle that were caught between converging plates and wedged upwards onto the continents. The rock sequences of ophiolites include these layers as well as an underlying layer of ultramafic rock that probably represents the upper mantle (see Figure 18.24 of the textbook, which compares oceanic crust and an ophiolite sequence).

The mid-oceanic ridge—a gigantic undersea mountain range, more than 80,000 km long and 1500 to 2000 km wide (described in detail in Chapter 18)—is of prime importance to the plate tectonic theory. This ridge rises two to three kilometres above the ocean floor, and except in the Pacific Ocean, the rift valley runs down the crest of the ridge. Tensional cracks that run parallel to the axis occur within the rift valley. From observations made by geologists from small research submarines in 1974, tensional cracking of the rift appears to be continuous, and sporadic volcanic activity occurs as a result of the rifting. The high heat flow associated with the ridge is caused by the rise of hot mantle rock and by basaltic eruptions.

To be considered valid, the theory of plate tectonics must be able to explain seamounts, guyots, and aseismic ridges (other features of the sea floor—see Chapter 18). Seamounts are conical undersea mountains that rise 1000 m or more above the sea floor. Since rocks dredged from seamounts are almost always basalt, seamounts are thought to represent extinct volcanoes. Guyots are flat topped seamounts. The flat tops are thought to have been cut by wave action before the guyots subsided to the depth where they are typically found. Aseismic ridge is the name given to a chain of seamounts and guyots aligned on the sea floor. Aseismic ridges are distinguished from the mid-oceanic ridge by their lack of earthquake activity.

The reading for this section describes magnetic anomalies in detail. Pay close attention to the Vine-Matthews hypothesis, and ensure that you can explain why it is so important in plate tectonic theory.

The reading also describes the origin of a diverging boundary on a continent and the developments of such a boundary as it continues to the sea floor. You may wish to outline the steps in the three major stages of this process.

Reading Assignment

Plummer, C. C., Carlson, D. H., & Hammersley, L. Physical Geology (15th ed.).

• “Seafloor Spreading” (pp. 460-461).
• “Plates and Plate Motion” (pp. 462).
• “How Do We Know That Plates Move?” (pp. 462-466).
• “Divergent Plate Boundaries” (pp. 466-470).

Study Questions

6. Briefly describe the dimensions of the mid-oceanic ridge.
7. Describe the three layers that make up the oceanic crust.
8. What are ophiolites?
9. Describe how basalt magma is formed at spreading centres.
10. What did Hess propose as the driving mechanism for seafloor spreading?
11. Briefly describe the explanation developed by Vine and Matthews for magnetic anomalies on the sea floor.
12. At what range of rates is the sea floor moving? How was this range determined?
13. How can the hypothesis of seafloor motion and the Vine-Matthews hypothesis about the origin of magnetic anomalies be tested?
14. Draw and label a diagram to illustrate the two hypotheses that were tested by the study of seismicity of fracture zones associated with the mid-oceanic ridge. Draw another diagram to show the resulting conclusions about plate motion.
15. Describe three stages by which a divergent plate boundary located in the middle of the continent becomes a divergent plate boundary located in the middle of the sea floor.

 Answer Key

Section 3: Converging Plate Boundaries

There are three different types of boundary convergence: ocean-ocean convergence, ocean-continent convergence, and continent-continent convergence. The reading for this section describes the features of each type in detail. You may wish to outline the characteristics of each type of convergence as a study strategy.

Reading Assignment

Plummer, C. C., Carlson, D. H., & Hammersley, L. Physical Geology (15th ed.).

• “Convergent Plate Boundaries” (pp. 471-474).

Study Questions

16. What factor determines the spacing between the trench and island arc associated with a subduction zone?
17. In what situation does a subduction trench not have an island arc associated with it?
18. What is an accretionary wedge, and how does it form? What is the relationship of such a wedge to the forearc basin?
19. Explain the process of backarc spreading.

 Answer Key

Section 4: Transform Boundaries

A transform plate boundary occurs where adjacent plates slide horizontally past each other, neither creating nor destroying lithosphere. Transform boundaries occur along transform faults, where the movement is parallel to the direction of relative plate motion and produces only fracturing and seismic activity. Transform faults can connect two segments of a ridge, two segments of a trench, or a ridge and a trench.

Reading Assignment

Plummer, C. C., Carlson, D. H., & Hammersley, L. Physical Geology (15th ed.).

• “Transform Boundaries” (pp. 470).

Study Question

20. Draw a sketch of three different types of transform boundaries. Indicate the relative motions of each plate.

 Answer Key

Section 5: The Cause of Plate Motion

The readings for this section describe some of the theories developed to answer the following question that is still being asked about plate tectonics: What is the actual driving mechanism of plate motion? It seems certain that convection of the mantle causes the movement, but whether the convection is deep, shallow, or two-tiered within the mantle is still under debate. Another possibility for convection of the mantle that is gaining support is that convection occurs in the form of mantle plumes, which are narrow columns of hot mantle rock that rise and spread radially outward. As study continues and more data is obtained, researchers may need to develop yet another theory.

Reading Assignment

Plummer, C. C., Carlson, D. H., & Hammersley, L. Physical Geology (15th ed.).

• “Do Plate Boundaries Move?” (p. 474).
• “Can Plates Change in Size?” (p. 474).
• “The Attractiveness of Plate Tectonics” (p. 474-475).
• “What Causes Plate Motions?” (pp. 475-481).
• “A Final Note” (pp. 482-83).

Study Questions

21. List at least four features of the Earth that the plate tectonic theory explains.
22. List three possible driving mechanisms for plate motion, and outline the pros, cons, or both for each.
23. Explain four process that are thought to cause lithospheric plates to spread apart at divergent margins and sink at subduction zones.
24. Describe the theory of mantle plumes. How are plates thought to move by this mechanism?
25. How are mid-plate volcanoes, such as the Hawaiian Island chain, explained by the mantle plume theory?

 Answer Key

Unit 14 Self Test

You have now finished Unit 14, so please complete and submit the associated laboratory exercise and assignment. Instructions can be found in the Assignment Drop Boxes section of the course homepage.